Rare earth sintered magnets are relatively susceptible to corrosion, as a property of the rare earth compounds that are used as their main components. For this reason, covering layers are formed on rare earth sintered magnets to minimize the reduction in magnetic properties that occurs with corrosion. Such covering layers are made of various materials such as plating films or resin films, depending on the purpose of use and the desired properties of the rare earth sintered magnet.
For example, Patent document 1 proposes heat treatment of a rare earth sintered magnet to form an oxide protective layer on the surface, for improvement of the corrosion resistance of the rare earth sintered magnet.